Nylons are polyamides that are generally synthesized by the condensation polymerisation of a diamine with a dicarboxylic acid. Similarly, nylons may be produced by the condensation polymerisation of lactams. A ubiquitous nylon is nylon 6,6, which is produced by reaction of hexamethylenediamine (HMD) and adipic acid. Nylon 6 is produced by a ring opening polymerisation of caprolactam. Therefore, adipic acid, hexamethylenediamine and caprolactam are important intermediates in the production of nylons (Anton & Baird, Polyamides Fibers, Encyclopedia of Polymer Science and Technology, 2001).
Industrially, adipic acid and caprolactam are produced via air oxidation of cyclohexane. The air oxidation of cyclohexane produces, in a series of steps, a mixture of cyclohexanone (K) and cyclohexanol (A), designated as KA oil. Nitric acid oxidation of KA oil produces adipic acid (Musser, Adipic acid, Ullmann's Encyclopedia of Industrial Chemistry, 2000). Caprolactam is produced from cyclohexanone via its oxime and subsequent acid rearrangement (Fuchs, Kieczka and Moran, Caprolactam, Ullmann's Encyclopedia of Industrial Chemistry, 2000)
Industrially, hexamethylenediamine (HMD) is produced by hydrocyanation of C6 building block to adiponitrile, followed by hydrogenation to HMD (Herzog and Smiley, Hexamethylenediamine, Ullmann's Encyclopedia of Industrial Chemistry, 2012).
Given a reliance on petrochemical feedstocks; biotechnology offers an alternative approach via biocatalysis. Biocatalysis is the use of biological catalysts, such as enzymes, to perform biochemical transformations of organic compounds.
Both bioderived feedstocks and petrochemical feedstocks are viable starting materials for the biocatalysis processes.
Accordingly, against this background, it is clear that there is a need for sustainable methods for producing adipic acid, caprolactam, 6-aminohexanoic acid, hexamethylenediamine and 1,6-hexanediol (hereafter “C6 building blocks”) wherein the methods are biocatalyst based (Tang et al., Biotechnology & Bioengineering, 2012, 109(10), 2437-2459).
However, no wild-type prokaryote or eukaryote naturally overproduces or excretes C6 building blocks to the extracellular environment. Nevertheless, the metabolism of adipic acid and caprolactam has been reported (Ramsay et al., Appl. Environ. Microbiol., 1986, 52(1), 152-156; and Kulkarni and Kanekar, Current Microbiology, 1998, 37, 191-194).
The dicarboxylic acid, adipic acid, is converted efficiently as a carbon source by a number of bacteria and yeasts via β-oxidation into central metabolites. β-oxidation of adipate to 3-oxoadipate faciliates further catabolism via, for example, the ortho-cleavage pathway associated with aromatic substrate degradation. The catabolism of 3-oxoadipyl-CoA to acetyl-CoA and succinyl-CoA by several bacteria and fungi has been characterised comprehensively (Harwood and Parales, Annual Review of Microbiology, 1996, 50, 553-590). Both adipate and 6-aminohexanoate are intermediates in the catabolism of caprolactam, finally degraded via 3-oxoadipyl-CoA to central metabolites.
Potential metabolic pathways have been suggested for producing adipic acid from biomass-sugar: (1) biochemically from glucose to cis,cis muconic acid via the ortho-cleavage aromatic degradation pathway, followed by chemical catalysis to adipic acid; (2) a reversible adipic acid degradation pathway via the condensation of succinyl-CoA and acetyl-CoA and (3) combining β-oxidation, a fatty acid synthase and ω-oxidation. However, no information using these strategies has been reported (Jang et al., Biotechnology & Bioengineering, 2012, 109(10), 2437-2459).
The optimality principle states that microorganisms regulate their biochemical networks to support maximum biomass growth. Beyond the need for expressing heterologous pathways in a host organism, directing carbon flux towards C6 building blocks that serve as carbon sources rather than as biomass growth constituents, contradicts the optimality principle. For example, transferring the 1-butanol pathway from Clostridium species into other production strains has often fallen short by an order of magnitude compared to the production performance of native producers (Shen et al., Appl. Environ. Microbiol., 2011, 77(9), 2905-2915).
The efficient synthesis of the six carbon aliphatic backbone precursor is a key consideration in synthesizing C6 building blocks prior to forming terminal functional groups, such as carboxyl, amine or hydroxyl groups, on the C6 aliphatic backbone.